NY unleashes predator bugs to eat hemlock pests

By Gwendolyn Craig

Nicholas Dietschler and Marshall Lefebvre were in a boat headed out to Lake George’s Paradise Bay, a cooler set between them.

They were not out for a picnic.

On this sunny, blustery May day, the pair of researchers from Cornell University’s New York State Hemlock Initiative were headed to the lake’s eastern shores to help save the most common shoreline tree in the watershed—hemlocks.

Last summer, a camper discovered the Adirondack Park’s second known infestation of the invasive hemlock woolly adelgid, on Lake George in Washington County. The state Department of Environmental Conservation has identified some 250 acres of infected trees, and in some areas like Shelving Rock Bay and Paradise Bay, the trees are so saturated with these invasive bugs that they’ve likely been there for six or more years.

Hemlocks are the fourth most populous tree in the Adirondack Park, and woolly adelgids are so small they can spread easily on the wind. The DEC continues to survey, but getting to remote locations and spotting infestations early is difficult, to say the least.

So the state is using all of its tools in the toolbox—or in the cooler—to treat the trees on Lake George. In addition to applying and injecting insecticides on the trees, the DEC is coordinating the introduction of three new species on Lake George, all of them bugs that love to eat adelgids.

Inside Dietschler and Lefebvre’s cooler were dozens of vials of what looked like fruit flies. They were two species of silver fly, native to the Pacific Northwest and predators of the hemlock woolly adelgid. Researchers raise these bugs in the initiative’s lab before releasing them in areas heavily infested with the adelgid.

“There’s so much adelgid, it wraps right around,” Dietschler said, observing white woolly balls dotted along lakeshore hemlock branches. “I don’t think we can go wrong here.”

The adelgids are all females that reproduce asexually. They colonize hemlock branches. Adults insert their mouthparts into the hemlock twigs, indirectly killing the buds. It takes years for a tree to show signs of infestation. In the winter, the adelgids shoot a waxy wool out their pores and huddle in the white balls, which Dietschler could easily see.

Unscrewing a vial, Dietschler flipped the opening upside down onto a hemlock branch, tapping the jar to get stragglers out. The idea is for the flies to breed and lay eggs, producing larvae to feed on the eggs of the adelgids. Dietschler, Lefebvre and DEC Forester Bryan Ellis went on to release about 1,400 flies.

While explaining that the bugs could be a longer-term solution to replace insecticide applications, Dietschler stopped short and gasped.

“Oh my gosh—those are mating,” he said, pointing to a hemlock twig. “There’s two flies mating, which we only observe in the lab like every once in a while.”

Ellis came over to see. One fly was leaned up against another, still. Nature was at work.

“That’s good news,” Dietschler said.

Last fall, the initiative and DEC released a third predator of HWA, the Laricobius nigrinus, around the same place. It’s a kind of beetle, also from the Pacific Northwest. Now, it’s a waiting game to see if the beetles and flies will establish populations and eat the adelgids. Researchers will come back next year and collect hemlock branches to see if they carry the DNA of these three predator bugs.

“The flies are small, but they are prolific poopers,” Lefebvre said. “That’s really what we’re hoping to capture, the DNA in their feces.”

That will be when they know if the experiment has worked.

Darr and Dietschler also understand some members of the public may be hesitant about the state using insecticides. DEC is specifically using imidacloprid and dinotefuran. For the trees closest to water and sensitive areas, the chemicals are injected into the tree so there’s no runoff contamination. The benefits outweigh Darr and Dietschler’s lack of enthusiasm for chemical applications.

“This is one of these pests where you really throw a few different control measures at a single problem,” Darr said.

Besides being the predominant tree along Lake George, hemlocks play an important role in curtailing erosion and keeping streams shaded and cool. State officials have called the trees essential for “vital ecosystem services.”

The same kinds of flies released on Lake George were also released about 95 miles south just a month before at the Edmund Niles Huyck Preserve in Rensselaerville. A hemlock woolly adelgid infestation was first spotted on the 2,000-acre preserve in 2015, and it reached a high infestation level by 2019. The pest was also spotted in Moreau Lake State Park in Saratoga County.